The present invention relates to a method and apparatus for correcting the asymmetry of digital signals and, more specifically, digital read signals in a read channel of a disc drive.
Use of magnetic media for mass storage of digital data in a computer system is widespread. Digital data is generally stored on a magnetic medium in the form of magnetic polarity inversions induced into the surface of the medium. If the medium is a magnetic disc, for example, the data is usually arranged in a series of concentric annuluses on the disc""s surface, known as tracks. To read data from one of these tracks, the disc is rotated at a constant speed, and a magnetic transducer is brought near the rotating track to convert the alternating magnetic field emanating from the track surface into an analog electrical signal. One type of magnetic transducer, which is widely used for reading digital data from a magnetic medium, is a magneto-resistive (MR) head.
An MR head is a device whose resistance varies with the applied magnetic field. In this regard, the head is capable of converting magnetic field variations produced by a rotating track into a time varying voltage or current in an electrical circuit. MR heads offer many advantages over other types of magnetic transducers and, consequently, are increasingly being used in magnetic data storage systems. For example, MR heads are more sensitive than other types of read heads, such as thin film heads, and produce a stronger read signal. Also, MR heads have a better frequency response than other types of heads which use inductive coils as a sensing means. In addition, the read signal produced by an MR head is relatively insensitive to the relative velocity between the head and the medium, as is the case with other types of heads, because it is the level of the applied magnetic field which is sensed by an MR head and not the rate of change of magnetic flux lines through a coil. This is an advantage in systems where head/medium velocity may vary over a significant range. Lastly, because MR heads are not capable of writing data on a magnetic medium, magnetic data storage systems which use MR read heads must include a separate head to perform the write function. Using a separate head for reading and writing allows each head to be separately optimized for performing its singular task which can greatly improve the performance of a magnetic data storage system.
As illustrated in FIG. 1, the relationship between the resistance, represented by the y-axis 100, of an MR head and the applied magnetic field, represented by the x-axis 101, is nonlinear. This nonlinear characteristic can produce problems in the conversion of the magnetic field variations emanating from the medium into the time varying electrical signal. For example, the nonlinear nature of the MR head may cause the time varying read signal produced by the MR head to look nothing like the magnetic signal applied to the head. To overcome this problem, a bias current is generally applied to the head to move the quiescent operating point of the head to a more linear region of the resistance characteristic. With reference to FIG. 1, it is seen that maximum linearity in the operation of an MR head is obtained by biasing the head at point 102, i.e., the most linear point on the characteristic. It may be desirable, however, to bias the head at another point, such as point 104 or point 106, to maximize a conversion parameter which may be more important than linearity, such as signal to noise ratio (SNR). As a consequence of such biasing, the output signal of the head may be asymmetrical about a zero volt baseline, such as output waveform 108 in FIG. 1 corresponding to bias point 104. In addition to biasing effects, other factors may also exist which result in an asymmetrical read signal, such as off-track effects.
In a disc drive using a partial response maximum likelihood (PRML) channel, the MR head is typically biased such that its output is asymmetric about a zero volt baseline. The difference in the magnitudes of the positive and negative peaks of the read signal complicates the sampling of the signal which must be performed before maximum-likelihood detection can occur. The asymmetry of the read signal leads to a higher mean square error in the detection stage of the PRML channel resulting in a higher error rate. As a result, it is advantageous to correct the asymmetry of a read signal prior to the detection stage of the PRML channel.
The amount of asymmetry of the read signal can be defined as the ratio of the larger peak magnitude of one polarity to the smaller peak magnitude of the other polarity. For example, the amount of asymmetry of the read signal of FIG. 1 would be approximately 1.2-0.7 resulting in an asymmetry of 1.7 (1.2÷0.7).
U.S. Pat. No. 5,744,993 (Sonntag) uses a technique called xe2x80x9cnonlinearity cancellationxe2x80x9d to compensate an asymmetric read signal. This non-linearity cancellation method believes that the asymmetric signal is a combination of the fundamental and second order of the input signal, and to get rid of the asymmetry, the second order of the input signal is removed. However, this method is only effective when the assumption that the asymmetry can be modeled by the second order of the signal is true. Furthermore, when the asymmetry is large enough such that the noise becomes significant, this method becomes ineffective. Currently, the maximum asymmetry that can be compensated using this method is approximately 1.5 or 150%.
The need therefore exists for a method and apparatus which are capable of overcoming the above-mentioned problems to compensate or correct an asymmetric read signal that is produced by an MR head.
The present invention relates correcting or compensating an asymmetric read signal in a read channel of a disc drive.
One embodiment of the invention is directed to a method of correcting digital samples of an asymmetric read signal. In accordance with this embodiment, a digital sample of the asymmetric read signal is read and a determination is made as to whether the digital sample requires compensation. If the digital sample requires compensation, a compensated sample can be generated. Finally, either the compensated sample or the digital sample can be outputted.
Another embodiment of the present invention is directed to an asymmetry correction block that is capable of implementing the above-mentioned method. One embodiment of the asymmetry correction block includes an input, a level detector, a compensator, and an output. The input receives a digital sample of the asymmetric read signal and provides the digital sample to the level detector and the compensator. The level detector determines whether the digital sample requires compensation. If it is determined that the digital sample requires compensation, the compensator generates a compensated sample. The output is configured to selectively output either the digital sample or the compensated sample. In one embodiment of the invention, the asymmetry correction block further includes a delay circuit. The delay circuit receives the digital sample from the input and delays providing it to the output to ensure proper synchronization with the compensator.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.